Tyrosine kinase inhibitors (TKIs) are a class of anti-cancerous monoclonal antibody drugs that target tumor growth, such as metastasized colorectal cancers (mCRC). TKIs are designed to block receptors and stop the growth signals that fuel the tumor thereby stopping the growth of the cancer cells.
The family of Ras genes encodes small GTPases that are involved in cellular signal transduction. Mutations in Ras genes can permanently activate the genes and cause inappropriate transmission inside the cell in the absence of extracellular signals. Because the signals result in cell growth and division, dysregulated Ras signaling can ultimately lead to oncogenesis and cancer. The Ras genes encode the Ras superfamily of proteins, which includes the KRAS (Kirsten rat sarcoma viral oncogene homolog) protein, which is encoded by the KRAS gene.
KRAS gene mutations are common in pancreatic cancer, lung adenocarcinoma, colorectal cancer, gall bladder cancer, thyroid cancer, and bile duct cancer. The status of KRAS mutations have been reported as predictive markers of tumor response to epidermal growth factor receptor (EGFR) TKI-targeted therapies; accordingly, the mutational status of KRAS can provide important information prior to the prescription of TKI therapy.
The most common KRAS mutations occur in codons 12 and 13 of exon 2. Other more rarely occurring mutations have been seen in codons 59 and 61 of exon 3. KRAS mutations at codons 12, 13, or 61 have been found to cause Ras proteins to remain longer in their active form, resulting in an over-stimulation of the EGFR pathway; consequently, patients with KRAS mutations at codons 12, 13, or 61 do not respond well to TKI therapy. Further, mutations in KRAS codon 12 or 13 have been shown to be strong predictors of patient non-responsiveness to anti-EGFR monoclonal antibody therapies, such as ERBITUX® (cetuximab; ImClone Systems Inc., New York, N.Y., USA) and VECTIBIX® (panitumumab, Amgen, Thousand Oaks, Calif., USA) for the treatment of certain cancerous conditions, including metastatic colorectal cancer (mCRC) and lung cancer. Massarelli et al., KRAS Mutation is an Important Predictor of Resistance to Therapy with Epidermal Growth Factor Receptor Tyrosine Kinase Inhibitors in Non-Small Cell Lung Cancer, CLIN CANCER RES. 13(10):2890-2896 (2007); Amado et al., Wild-type KRAS is Required for Panitumumab Efficiency in Patients with Metastatic Colorectal Cancer, J. CLIN ONCOL 26(10):1626-1634 (2008); Van Cutsem et al., KRAS Status and Efficacy in the First-Line Treatment of Patients with Metastatic Colorectal Cancer (mCRC) Treated with FOLFIRI with or without Cetuximab: The CRYSTAL Experience, J CLIN ONCOL 26(15S): May 20 Supplement, Abstract 2 (2008); Baker et al., Evaluation of Tumor Gene Expression and KRAS Mutations in FFPE Tumor Tissue as Predictors of Response to Cetuximab in Metastatic Colorectal Cancer, J CLIN ONCOL 26(15S): May 20 Supplement, Abstract 3512 (2008); Van Zakowski et al., Reflex Testing of Lung Adenocarcinomas for EGFR and KRAS Mutations: The Memorial Sloan-Kettering Experience, J. CLIN ONCOL 26(15S): May 20 Supplement, Abstract 22031 (2008).
On Jul. 20, 2009, the FDA updated the labels of ERBITUX® and VECTIBIX® to include the following statements specific to the use of the drugs for mCRC treatment on the “Indications and Use” sections of the respective labels for the drugs:
“Retrospective subset analyses of metastatic or advanced colorectal cancer trials have not shown a treatment benefit for Erbitux in patients whose tumors had KRAS mutations in codon 12 or 13. Use of Erbitux is not recommended for the treatment of colorectal cancer with these mutations [see Clinical Studies (14.2) and Clinical Pharmacology (12.1)].” ERBITUX® label, Indications and Usage (as of Jul. 20, 2009).
“Retrospective subset analyses of metastatic colorectal cancer trials have not shown a treatment benefit for Vectibix in patients whose tumors had KRAS mutations in codon 12 or 13. Use of Vectibix is not recommended for the treatment of colorectal cancer with these mutations [see Clinical Studies (14) and Clinical Pharmacology (12.1)].” VECTIBIX® label, Indications and Usage (as of Jul. 20, 2009).
One currently used method for detecting KRAS codon 12 and 13 mutations is direct sequencing. A major limitation with using direct sequencing is the sensitivity of the assay. The assay is typically run with a small proportion of tumor/normal cells obtained from a patient biopsy. Direct sequencing will detect a minority population only when it is present in a concentration of approximately 15% or greater.
Another method for detecting KRAS codon 12 and 13 mutations is the commercially available THERASCREEN® KRAS Mutation Test Kit (DxS Limited, Manchester, UK). One disadvantage of the THERASCREEN® KRAS Mutation Test Kit method is that the algorithm used to genotype the DNA sample is different for each of the assays and the cycle threshold (Ct) cutoff value for each assay is variable.
In addition to data that supports the fact that patients with mCRC that have KRAS mutations are clinically resistant to therapy with anti-EGFR monoclonal antibodies, there is also data to suggest that mutations that activate PIK3CA (phosphatidyl inositol 3-kinas catalytic subunit) genes are also associated with resistance to anti-EGFR monoclonal antibodies. Sartore-Bianchi et al., PIK3CA Mutations in Colorectal Cancer are Associated with Clinical Resistance to EGFR-Targeted Monoclonal Antibodies, CANCER RES. 69(5):1851-1857 (2009).
The PIK3CA gene encodes for a lipid kinase that together with KRAS regulates signaling pathways downstream of the EGFR. It is not currently known to what extent the occurrence of PIK3CA mutations affect the responsiveness of patients with mCRC, or other cancers, to anti-EGFR monoclonal antibodies; however the literature suggests that the PIK3CA gene is mutated on average in 15% of human cancers over vast range of tissue types. Karakas, et al., Mutation of the PIK3CA Oncogene in Human Cancers, BRITISH J CANCER 94(4):455-459 (2006); Li et al., Mutations of PIK3CA in Gastric Adenocarcinoma, BIOMED CENTRAL CANCER 5:29 (2005); Qiu et al., PIK3CA Mutations in Head and Neck Squamous Cell Carcinoma, CLIN CANCER RES. 12(5):1441-1446 (2006). The most frequent PIK3CA mutations are E542K (Glu524Lys), E545K (Glu545Lys), and E545D (Glu545Asp) mutations in exon 9 and H1047R (His1047Arg) mutations in exon 20.